Under a 2015 nuclear deal between Iran and world powers, the Islamic republic agreed to modify the Arak reactor so that it could not produce military-grade plutonium.

"This project must be reconfigured and returned to operation as soon as possible," Iranian media on Saturday quoted Mohammad Eslami as saying during a visit to the site this week.

No time frame was specified.

The nuclear deal gave Iran sanctions relief in return for tight controls on its nuclear programme, monitored by the UN.

Tehran has gradually rolled back its nuclear commitments since 2019, a year after then US president Donald Trump withdrew from the multilateral deal and began reimposing sanctions.

Iran said in 2019 that a secondary circuit for the Arak reactor had become operational as part of its redesign, but that the reactor's primary circuit, which contains the core, was still being built.

It also said that the US withdrawal from the nuclear accord had slowed the reactor's conversion.

Eslami's comments came just days after the head of the UN's nuclear watchdog, Rafael Grossi, visited Tehran and reached a temporary arrangement to continue surveillance of Iranian nuclear facilities.